This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for developing electrostatic latent images recorded on a photoconductive member.
In general, an electrophotographic printing machine comprises a photoconductive member which is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image corresponding to the informational areas contained within the original document, on the photoconductive member. The electrostatic latent image is developed by bringing a developer mix into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. Finally, the copy sheet is heated to permanently fix the powder image thereto in image configuration.
A suitable developer mix comprises toner particles adhering triboelectrically to carrier granules. This two component mixture is brought into contact with the photoconductive surface. The toner particles are attracted from the carrier granules to the latent image and, to some extent to the non-image or background areas. Those particles adhering to the latent image form a powder image on the photoconductive surface. Hereinbefore, it has been difficult to develop both the large solid regions of the latent image and the lines thereof without developing the background regions. Frequently, solid area development resulted in the background areas attracting the toner particles thereto. Ultimately, the toner particles, in this unwanted or background region, are transferred to the copy sheet resulting in a degradation in copy quality. Different techniques have been employed in attempting to improve solid area development without developing the unwanted background regions. For example, a development electrode or screening technique is frequently employed to improve solid area development while suppressing development of the background areas which have a lower potential than the solid areas. However, these systems are all rather complex and have suffered from poor development latitude resulting in low density images being formed on the copy sheets.
Various approaches have been devised to improve development. The following disclosures appear to be relevant:
U.S. Pat. No. 3,176,652; Patentee: Mott et al.; Issued: Apr. 6, 1965.
U.S. Pat. No. 3,608,522; Patentee: Davidson; Issued: Sept. 28, 1971.
U.S. Pat. No. 3,950,089; Patentee: Fraser et al.; Issued: Apr. 13, 1976.
U.S. Pat. No. 4,086,873; Patentee: Morita et al.; Issued: May 2, 1978.
Japanese Patent Application No. 52-100746; Application Date: Aug. 22, 1977.
Japanese Laid Open No. 54-34244; Laid Open Date: Mar. 13, 1979; Applicant: Minolta Camera Company, Ltd..
Co-pending U.S. Application Ser. No. 63,893; Applicant: Burnham et al.; Filed: Aug. 3, 1979.
The relevant portions of the foregoing disclosures may be briefly summarized as follows:
Mott et al. describes a developing apparatus comprising an elongated magnet disposed interiorly of a rotatably mounted cylindrical shield. The shield is non-magnetic and also may be electrically insulating.
Davidson describes a pair of magnetic rollers. Each magnetic roller comprises an outer cylinder of non-magnetic material with an elongated bar magnet being disposed interiorly of each cylinder.
Fraser et al. discloses a magnetic brush development system having a rotatably driven applicator roll. As shown in FIG. 3, the applicator roll includes a magnet disposed within a conductive sleeve coated or held in intimate contact with a sheet of highly resistive material.
In FIG. 3, Morita et al. shows a magnetic brush development system comprising an electrically conductive cylindrical member having a layer of highly electrically insulating material coated thereon. The resistivity of the insulating layer ranges from about 10.sup.8 to about 10.sup.15 ohms per centimeter.
The Japanese patent application discloses a development system including a magnetic roll disposed interiorly of a sleeve. The sleeve is made from a double layered structure with the outer layer being a non-magnetic conductive cylinder and the inner-layer being a non-magnetic insulating member.
The co-pending U.S. patent application describes a conductive magnetic brush roller and an insulating brush roller. The conductive magnetic brush roller includes a non-magnetic, conductive tubular member having an elongated magnet disposed interiorly thereof. The insulating magnetic brush roller includes an insulating, non-magnetic, tubular member having a magnet disposed interiorly thereof. The insulating tubular member is preferably made from a phenolic material.